The electromotive force (EMF) is the maximum potential difference between two electrodes of a galvanic or voltaic cell, where the standard hydrogen electrode is on the left-hand side for the following cell:
12Pt ElectrodeH2Aqueous Electrolyte10−3 M Fe(CIO4)3PtSolution10−3 M Fe(CIO4)2The EMF is called the electrode potential of the electrode placed on the right-hand side in the graphical scheme of the cell, but only when the liquid junction between the solutions can be neglected or calculated, or if it does not exist at all.
The electrode potential of the electrode on the right-hand side (often called the oxidation-reduction potential) is given by the Nernst equationEFe3+/Fe2+=EFe3+/Fe2+0+(RT/F)ln(aFe3+/aFe2+)
This relationship follows from equation (2.21) when (μFe3−0−μFe2+0)/F is set equal to EFe3+/Fe2+0 and the pH and ln pH2 are equal to zero. In the subscript of the symbol for the electrode potential the symbols for the oxidized and reduced components of the oxidation-reduction system are indicated. With more complex reactions it is particularly recommended to write the whole reaction that takes place in the right-hand half of the cell after symbol E (the ‘half-cell’ reaction); thus, in the present caseEFe3+/Fe2+≡E(Fe3++e=Fe2+)
Quantity EFe3+/Fe2+0 is termed the standard electrode potential. It characterizes the oxidizing or reducing ability of the component of oxidation-reduction systems. With more positive standard electrode potentials, the oxidized form of the system is a stronger oxidant and the reduced form is a weaker reductant. Similarly, with a more negative standard potential, the reduced component of the oxidation-reduction system is a stronger reductant and the oxidized form a weaker oxidant.
The standard electrode E0, in its standard usage in the Nernst equation, equation (1-2) is described as:
  E  =            E      0        +                            2.3          ⁢                                          ⁢          RT                nF            ⁢                          ⁢      log      ⁢                          ⁢                                    C            0                    ⁡                      (                          0              ,              t                        )                                                C            R                    ⁡                      (                          0              ,              t                        )                              where E0 is the standard potential for the redox reaction, R is the universal gas constant (8.314 JK−1 mol−1), T is the Kelvin temperature, n is the number of electrons transferred in the reaction, and F is the Faraday constant (96,487 coulombs). On the negative side of E0, the oxidized form thus tends to be reduced, and the forward reaction (i.e., reduction) is more favorable. The current resulting from a change in oxidation state of the electroactive species is termed the faradaic.
Previous work describes using conversion of functional groups attached to a transitional metal complex resulting in quantifiable electrochemical signal at two unique potentials, E°1 and E°2. See for example, U.S. Patent Publication Nos. US 2011 0033869 and US 2012-0181186, all herein incorporated by reference in their entirety. The methods generally comprise binding an analyte within a sandwich of binding ligands which may have a functional tag, on a solid support other than the electrode. After target binding, a peroxide generating moiety or an intermediary enzyme and substrate are added which generates hydrogen peroxide. The redox active complex is bound to an electrode and comprises a peroxide sensitive moiety (PSM). The peroxide generated from the enzyme system reacts with the PSM, removing a self-immolative moiety (SIM) and converting functional groups attached to a transitional metal complex resulting in quantifiable electrochemical signal at two unique potentials, E°1 and E°2.